The reason those samples end up being i5-6400 units rather than higher tiered models, is because they've shown to be poorly binned wafers. There is always a chance you could get one that slipped through the cracks just to fulfill supply, but generally speaking those models would have very little overclocking headroom and generally aren't worth the effort or expense to overclock because although they have fair single core boot speeds on paper, the fact is that they become very unstable with medium to high all core overclocks. Overclocking to 2.8Ghz from 2.7Ghz, does nothing. The boost speeds are higher than that. Honestly, those CPUs are a waste of time to bother with trying to configure high frequency overclocks on because the all core boost speeds are generally already as high as you'd get from a manual overclock anyhow.
If it was a higher binned sample, there would be more headroom, but there's not, which is why those are turned into inexpensive bottom end i5 samples.